Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ a fixing belt, formed into a loop, to apply heat to the recording medium bearing the toner image, and a pressing roller, disposed opposite the fixing belt, to apply pressure to the recording medium. A stationary, nip formation pad disposed inside the loop formed by the fixing belt is pressed against the pressing roller disposed outside the loop formed by the fixing belt via the fixing belt to form a fixing nip between the fixing belt and the pressing roller through which the recording medium bearing the toner image passes. As the fixing belt and the pressing roller rotate and convey the recording medium through the fixing nip, they apply heat and pressure to the recording medium to fix the toner image on the recording medium.
As a mechanism that heats the fixing belt, the fixing device may include an exciting coil disposed opposite the fixing belt, which generates a magnetic flux toward the fixing belt, thus heating a heat generation layer of the fixing belt by electromagnetic induction.
For example, Japanese publication No. P2009-282413A proposes a configuration in which a temperature-sensitive magnetic member, which generates heat by a magnetic flux generated by the exciting coil, separably contacts the inner circumferential surface of the fixing belt. Before the fixing belt is heated to a desired fixing temperature, the temperature-sensitive magnetic member is isolated from the fixing belt; therefore it does not draw heat from the fixing belt, shortening a warm-up time of the fixing belt. Conversely, after the fixing belt has been heated to the desired fixing temperature, the temperature-sensitive magnetic member contacts the fixing belt to conduct heat thereto supplementarily, thus maintaining the fixing temperature of the fixing belt.
However, such configuration has a drawback in that, even when the temperature-sensitive magnetic member is isolated from the fixing belt during warm-up, it is still heated by the magnetic flux generated by the exciting coil. That is, the magnetic flux is not concentrated solely on the fixing belt, thereby degrading heating efficiency for heating the fixing belt.
As another example, Japanese patent No. P3,527,442 proposes a configuration in which a conductive member is rotatably disposed inside a heating roller in such a manner that it is moved between the two positions: a first position where it is disposed opposite an exciting coil disposed outside the heating roller, and a second position where it is not disposed opposite the exciting coil. With this configuration, before the heating roller is heated to a desired fixing temperature, the conductive member is at the second position where it is not disposed opposite the exciting coil so that a magnetic flux generated by the exciting coil is concentrated solely on the heating roller, not reaching the conductive member. By contrast, after the heating roller has been heated to the desired fixing temperature, the conductive member is moved to the first position where it is disposed opposite the exciting coil.
However, such configuration also has a drawback in that the heating roller is constructed of a heat generation layer heated by the magnetic flux generated by the exciting coil and a temperature-sensitive magnetic layer, which prevents overheating of the heating roller, combined with the heat generation layer. Since the temperature-sensitive magnetic layer is combined with the heat generation layer, it draws heat from the heat generation layer, lengthening a warm-up time of the heating roller.